Preparation

Position a rack in the center of the oven and heat the oven to 375°F.

Scrub the potatoes and pat them dry. Put them on a baking sheet and lightly sprinkle with salt. Bake until tender when poked with a skewer, 40 to 50 minutes. Remove from the oven and set aside until cool enough to handle.

While the potatoes are cooling, heat 2 Tbs. of the olive oil in a 12-inch skillet over medium heat. Add the onion and a pinch of salt and cook, stirring often, until softened, about 5 minutes. Add the garlic and cook for 1 minute, then add the watercress. Season to taste with salt and pepper and cook, stirring, just until the watercress wilts, 1 to 2 minutes.

Put the potatoes in a large bowl and break them into small chunks with a spoon. Add the watercress mixture and the mascarpone, mix to combine, and season to taste with salt and pepper. Divide and shape the potato mixture into 16 equal patties, about 2 inches in diameter and ½ inch thick. (The cakes can be made to this point up to 4 hours ahead and refrigerated.)

Heat the oven to 200°F. Lightly oil a rimmed baking sheet. Heat the remaining 3 Tbs. olive oil in a 12-inch nonstick skillet over medium heat. Working in 2 batches, cook the potato cakes until golden on both sides, 3 to 4 minutes per side. Transfer each batch to the baking sheet, season lightly with salt, and keep warm in the oven for up to 30 minutes before serving.